Prognostic Meteorological Discussion
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000
FXUS02 KWBC 150701
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 AM EST FRI DEC 15 2017

VALID 12Z MON DEC 18 2017 - 12Z FRI DEC 22 2017

...OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT...


SHORTWAVE ENERGY SPLITS OVER THE WRN US TO CARVE OUT A WET SWRN US
MID-UPPER LEVEL TROUGH INTO MON THAT SHEARS PROGRRESSIVELY OUT
ACROSS THE US SRN TIER STATES EARLY-MID NEXT WEEK. MEANWHILE...A
SERIES OF NRN STREAM SYSTEMS PROGRESS OVERTOP THROUGH THE CENTRAL
AND ERN US NRN TIER STATES. THE 00 UTC GFS AND 12/00 UTC ECMWF
HAVE CONVERGED ON THEIR SOLUTIONS FOR THIS TIME
PERIOD...BOLSTERING FORECAST CONFIDENCE BACK TO AVERAGE
LEVELS...ALBEIT WITH SOME PESKY CONTINUED EMBEDDED SYSTEM TIMING
VARIANCES. A BLEND OF THE 00 UTC GFS/ECMWF AND PERHAPS A BIT OF
SMOOTHING ENSEMBLES SEEMS REASONABLE.

FORECAST SPREAD AND UNCERTAINTY INCREASE MID-LATER NEXT WEEK AS
GUIDANCE STRUGGLES WITH THE EXTENT/DETAILS OF ADDITIONAL POTENT
SHORTWAVE TROUGH ENERGY DIGGING FROM THE ERN PACIFIC DOWN THROUGH
THE WRN STATES IN RESPONSE TO SHARP UPSTREAM MID-UPPER RIDGE
AMPLIFICATION. THE 00 UTC GFS HAS ABANDONED THE DEVELOPMENT OF A
SWRN US CLOSED LOW LATE NEXT WEEK CHAMPIONED BY EARLY GFS RUNS.
HOWEVER...SUPPORTING TROUGH DIGGING THROUGH THE WEST COAST INTO
THE SWRN US MAY STILL BE AMPLE SO USED THE MOST AMPLIFIED OF THE
ENSEMBLE MEANS...THE ECMWF ENSEMBLES...INTO DAY 6/7 GIVEN UPSTREAM
SUPPORT FROM SUCH A AMPLIFIED UPSTREAM RIDGE ALOFT OVER THE
PACIFIC. THIS ENERGY IS THEN INJECTED INTO/UNDERNEATH A BROAD
LONGWAVE THROUGH OVER THE CENTRAL TO ERN US.


...WEATHER HIGHLIGHTS/THREATS...


IT REMAINS THE CASE THAT THE GULF COAST/SRN TIER STATES WILL
REMAIN AN AREA OF INTEREST FOR HEAVY RAINFALL POTENTIAL WITH
EJECTING W-CENTRAL U.S. SHORTWAVE ENERGY/ASSOCIATED COLD FRONT AS
WELL AS A POSSIBLE LEADING STATIONARY BOUNDARY PROVIDING FOCUS FOR
THIS ACTIVITY. BEST POTENTIAL FOR HIGHEST RAINFALL TOTALS APPEARS
TO BE SUN-TUE...THEN OUT ACROSS THE ERN STATES. MEANWHILE...THE
APPROACH/ARRIVAL OF THE NORTHEAST PACIFIC UPPER TROUGH WILL LIKELY
BRING A PERIOD OF MEANINGFUL RAIN/HIGHER ELEVATION SHOW TO FAVORED
TERRAIN OF THE PACIFIC NORTHWEST AND NRN ROCKIES. PRECIP SHOULD BE
CONFINED TO FAR NRN AREAS EARLY NEXT WEEK AND THEN PUSH SOUTHWARD
MID-LATER WEEK WITH SOME MOISTURE WORKING MORE INTO CA. THE
POTENTIAL FOR THE COMBINATION OF SNOW AND WIND WILL HAVE TO BE
MONITORED OVER PARTS OF THE ROCKIES/PLAINS AS LOW PRESSURE EMERGES
INTO THE CENTRAL U.S. LATER WEEK. OVER THE EAST...MOST SNOW SHOULD
BE CONFINED TO NRN TIER LOCATIONS WITH A COMBINATION OF
SYNOPTIC/LOW PASSAGE AND LAKE EFFECT ACTIVITY.

SCHICHTEL/RAUSCH

$$




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